1. Field of the Invention
This invention relates generally to fluid pressure control valves and more particularly to improvements in valves of the class having a valve member which is positioned by the opposing forces of a spring and fluid pressure.
2. Discussion of the Prior Art
This invention is primarily concerned with pressure relief valves for hydraulic systems, especially relief valves of the kind disclosed in prior U.S. Pat. No. 2,917,072, and will be described in the context of such valves. It will become evident as the description proceeds, however, that the improvements of the invention may be used to advantage in other valves of the class described.
Relief valves in general are very old in the art and widely used to limit maximum fluid pressure in fluid handling systems and the like. A typical relief valve has a body containing a fluid inlet, a fluid outlet, a fluid passage communicating the inlet and outlet, and a valve member or poppet which is spring biased toward a valve seat against the fluid pressure at the valve inlet. In use, the valve inlet is connected to a fluid pressure system, such as a hydraulic system, whose fluid pressure is to be limited, and the valve outlet is connected to a low pressure fluid receiver, such as hydraulic fluid reservoir. The poppet spring retains the poppet in a closed position wherein the poppet contacts the valve seat to block fluid flow through the valve so long as the inlet pressure remains below a certain pressure level. If the inlet pressure exceeds this pressure level, the poppet opens against spring force to vent fluid from the system.
The prior art is replete with a vast assortment of such relief valves. Prior U.S. Pat. No. 2,917,072, mentioned above, is concerned with reducing the spread between the cracking pressure and the full opening pressure of a relief valve, that is the spread between inlet pressure at which the valve cracks open just slightly and the inlet pressure at which the valve opens fully. The valve seat of this valve is movable to permit alignment of the seat and poppet and is spring biased toward the poppet. The poppet and valve seat springs normally yieldably retain the poppet and seat in closed positions wherein the poppet engages the seat to block fluid flow through the valve. In these closed positions, the seat has an unbalanced area exposed to fluid pressure at the valve inlet, whereby the seat is urged toward the poppet by both spring force and inlet fluid pressure.
Below a certain inlet fluid pressure level, referred to herein as a threshold pressure level, the poppet and valve seat are yieldably retained by their springs in certain normal closed positions wherein the poppet and seat occupy certain normal positions relative to the valve body and contact one another to block fluid flow through the valve. Increasing inlet pressure above this threshold pressure level moves the seat and poppet as a unit from such normal positions in the flow direction through the valve while the poppet and seat remain in mutual seating contact to block flow through the valve. This movement of the poppet and valve seat as a unit by increasing inlet pressure while blocking fluid flow through the valve continues until the seat engages a limit stop on the valve body at some elevated inlet pressure level higher than the threshold pressure level. Any further increase in inlet pressure then moves the poppet out of engagement with the seat to first crack the valve open and then immediately open the valve fully in such a way that the spread between the initial cracking pressure of the valve and its full opening pressure is relatively small. Fluid flow through the open valve occurs from the inlet, which is located at one end of the valve body, to an outlet at the opposite end of the body.
Operation of the prior relief valve is enhanced by its incorporation of a dashpot for damping movement of the valve poppet. This dashpot comprises a post which is attached at one end to the valve body and has at its other end a piston which slides with a narrow clearance within a cylinder-forming recess or bore in the poppet. Movement of the poppet in the valve body results in relative movement of the piston in the cylinder to damp such poppet movement and thereby prevent unstable operation of the relief valve during periods of rapid fluctuations in the valve inlet pressure.
This prior valve has an inlet chamber upstream of the valve poppet and seat, and the dashpot post is attached to a spider-like support which spans this inlet chamber. The post extends from the spider through the inlet chamber and a central opening in the valve seat to the cylinder in the poppet. Incoming fluid flow occurs through openings in the supporting spider, then through the annular flow spaces about the dashpot post, and finally through generally longitudinal flow passages in the valve poppet to the valve outlet.
This valve arrangement is quite satisfactory for many applications but has certain disadvantages in other applications, notably those which require a relatively small overall valve size. This disadvantage resides in the fact that the dashpot (i.e., the dashpot post) is located in the path of fluid flow through the valve and thus obstructs fluid flow through the valve. The flow openings or passages in the post supporting spider and in the valve member further restrict flow through the valve. Accordingly, the prior valve arrangement restricts the flow capacity of the prior valve when fully open. In a relatively large size valve, of course, the flow passages can be made sufficiently large to provide a desired flow capacity. The valve size can be reduced only so much, however, without causing unacceptable flow restriction.